The endorsement comes as Trump is locked in a dead heat with Texas Sen. Ted Cruz in Iowa. The two have been ramping up their attacks against one another as the election has neared.

In the statement announcing the endorsement, Trump's campaign described Palin as a conservative who "helped launch the careers of several key future leaders of the Republican Party and conservative movement." The statement also quoted Cruz as once saying he "would not be in the United States Senate were it not for Gov. Sarah Palin. ... She can pick winners."

Sarah Palin and Donald Trump. Image courtesy: YouTube

"Media heads are spinning," Palin said after taking the stage at a Trump rally at Iowa State University. "This is going to be so much fun."

Palin, the former governor of Alaska and 2008 Republican vice presidential candidate, said that, with Trump as president, America would no longer apologise.

"No more pussy-footing around," Palin said.

Campaigning in New Hampshire, Cruz said, "Regardless of what Sarah intends to do in 2016, I will remain a big, big fan of Sarah Palin."

Palin endorsed Cruz in his 2012 Senate race in Texas and said as recently as last month that he and Trump were both in her top tier of candidates, making the endorsement a symbolic blow to Cruz who says Trump is not a true, consistent conservative and represents "New York values."

Palin's remarks in Ames, Iowa, were signature Palin, combining the folksy charm and everywoman appeal that initially made her a Republican superstar with defiant taunting of a "busted" party establishment that she slammed for counting both Trump and herself out.

Palin offered her full-throated support for Trump and slammed President Barack Obama as the "capitulator in chief." Trump, she said, would be a commander in chief who would "let our warriors do their job and go kick ISIS' ass!"

She also took aim at the Republican establishment for "attacking their own front-runner" and offered a challenge to those who have suggested that Trump, whose positions on issues like gun control and abortion rights have shifted over the years, isn't conservative enough.

"Oh my goodness gracious. What the heck would the establishment know about conservativism?" she said. "Who are they to tell us that we're not conservative enough? ... Give me a break."

Trump, whose team had been touting a major, surprise announcement, praised Palin as "a friend, and a high-quality person whom I have great respect for" in a statement.

"We're going to give' em hell," he said after her speech.

Palin will also be joining Trump at two events Wednesday, including a rally in Tulsa, Oklahoma.

Palin was a virtual newcomer to the national political arena when Arizona Sen. John McCain named her as his running mate. She has since risen to prominence as one of the most outspoken conservatives in the party. She signed on as a Fox News commentator after resigning as Alaska's governor in 2010, a job she held until last year.

Trump and Palin did not discuss how the endorsement had come about, but Trump's national political director Michael Glassner previously worked for her.

The event came a day after Palin's oldest son, Track, was arrested in Wasilla, Alaska, in a domestic violence case in which his girlfriend told police she was afraid he would shoot himself with a rifle. Track Palin was charged with assault, interfering with the report of a domestic violence crime and possessing a weapon while intoxicated in connection with the incident.

Earlier Tuesday, Trump received an endorsement from the daughter of movie star John Wayne.

Standing in front of a life-size, rifle-toting model of the actor in full cowboy gear, Trump accepted the endorsement of Aissa Wayne at the John Wayne Birthplace Museum in Winterset, Iowa.

"America needs help and we need a strong leader and we need someone like Mr. Trump with leadership qualities, someone with courage, someone that's strong, like John Wayne," she said.